Optical scanning device and color image forming apparatus

ABSTRACT

By a first image forming optical system ( 42   a  to  42   d ), a plurality of light beams from light sources ( 41   a  to  41   d ) form linear images on a common deflecting surface ( 46 ) of an optical deflector ( 44 ). Light beams reflected off the optical deflector ( 44 ) are reflected by a plurality of curved surface mirrors ( 45   a  to  45   d ) and are allowed to scan over photosensitive members ( 4   a  to  4   d ), respectively. The plurality of curved surface mirrors ( 45   a  to  45   d ) are disposed on the same side with respect to a plane that includes a normal line at a center of the deflecting surface ( 46 ) and is parallel to a main scanning direction. Further, curved surfaces of the plurality of curved surface mirrors ( 45   a  to  45   d ) vary in shape. Thus, a tandem type color image forming apparatus that achieves low cost and has excellent optical performance and an optical scanning device that is used favorably in the color image forming apparatus can be provided.

TECHNICAL FIELD

The present invention relates to a color image forming apparatus that istypified by a laser beam printer, a laser facsimile, a digital copierand the like and an optical scanning device for use in such color imageforming apparatuses.

BACKGROUND ART

As a conventional color image forming apparatus, for example, aso-called tandem type apparatus is known in which a plurality of imageforming units are arranged in order with respect to a paper-conveyingpath along a horizontal direction, and toner images are transferredsequentially from the image forming units onto a paper sheet beingconveyed along the paper-conveying path so that a color image is formedon the paper sheet. As an optical scanning device used in the tandemtype color image forming apparatus, a device merely using four opticalscanners with each scanning a single light beam (see JP2000-141759 A), adevice using a single optical deflector and four sets of lens systems(see JP2001-133717 A), or a device using four sets of curved surfacemirrors and lenses (see JP10(1998)-148777 A) is known.

However, the optical scanning devices proposed as above have presentedthe following problems. That is, the optical scanning devices require alarge number of components and thus are costly. Further, in the opticalscanners, uniform performance of scanning lines hardly can be provided.

DISCLOSURE OF THE INVENTION

In view of the above-mentioned problems, it is an object of the presentinvention to provide a tandem type color image forming apparatus thatachieves low cost and has excellent optical performance and an opticalscanning device that is used favorably in the color image formingapparatus.

In order to achieve the above-mentioned object, an optical scanningdevice according to the present invention includes: a plurality of lightsources; a single optical deflector that scans light beams emittedrespectively from the plurality of light sources; a first image formingoptical system that is disposed between the optical deflector and theplurality of light sources and allows linear images of the light beamsto be formed on a common deflecting surface of the optical deflector;and a second image forming optical system that is disposed between theoptical deflector and a plurality of surfaces to be scannedcorresponding to the plurality of light sources and has a plurality ofcurved surface mirrors that are in one-to-one correspondence with theplurality of surfaces to be scanned. In the optical scanning device, theplurality of light sources, the optical deflector, and the second imageforming optical system are disposed at different positions in asub-scanning direction so that light beams from the first image formingoptical system are incident respectively on the deflecting surfaceobliquely with respect to a plane that includes a normal line at acenter of the deflecting surface of the optical deflector and isparallel to a main scanning direction (hereinafter, referred to as a“main scanning plane”), and so that light beams from the opticaldeflector are incident respectively on the plurality of curved surfacemirrors obliquely with respect to a plane that includes each of normallines at vertices of the plurality of curved surface mirrors and isparallel to the main scanning direction. Further, in the opticalscanning device, the plurality of curved surface mirrors are disposed onthe same side with respect to the main scanning plane, and curvedsurfaces of the plurality of curved surface mirrors vary in shape.

Furthermore, a color image forming apparatus according to the presentinvention includes: the above-described optical scanning deviceaccording to the present invention; a plurality of photosensitivemembers that are disposed respectively on the plurality of surfaces tobe scanned; a plurality of developers that correspond respectively tothe plurality of photosensitive members and develop toner images ofdifferent colors respectively on the plurality of photosensitivemembers; a transferring unit that transfers the toner images on theplurality of photosensitive members onto a transfer material; and afixer that fixes the toner images transferred onto the transfermaterial.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic structural view of an optical unit that is anoptical scanning device according to Embodiment 1 of the presentinvention.

FIG. 2 is a front view of curved surface mirrors used in the opticalscanning device according to Embodiment 1 of the present invention.

FIG. 3 is a cross-sectional view taken on an XZ plane for showing curvedsurface mirrors used in an optical scanning device according toEmbodiment 2 of the present invention.

FIG. 4 is a front view of the curved surface mirrors used in the opticalscanning device according to Embodiment 2 of the present invention.

FIG. 5 is a schematic structural view of a first image forming opticalsystem according to Embodiment 3 of the present invention.

FIG. 6 is a schematic structural view of an optical unit that is anoptical scanning device according to Embodiment 4 of the presentinvention.

FIG. 7 is a schematic structural view of an optical unit that is anoptical scanning device according to Embodiment 5 of the presentinvention.

FIG. 8 is a schematic structural view of an optical unit that is anoptical scanning device according to Embodiment 6 of the presentinvention.

FIG. 9 is a schematic structural view of an optical unit that is anoptical scanning device according to Embodiment 7 of the presentinvention.

FIG. 10 is a cross-sectional view taken on an XZ plane for showingcurved surface mirrors used in the optical scanning device according toEmbodiment 7 of the present invention.

FIG. 11 is a cross-sectional view taken on a YZ plane for showing thecurved surface mirrors used in the optical scanning device according toEmbodiment 7 of the present invention.

FIG. 12 is a schematic structural view of an optical unit that is anoptical scanning device according to Embodiment 8 of the presentinvention.

FIG. 13 is a schematic structural view of a color image formingapparatus according to Embodiment 9 of the present invention.

FIG. 14 is a cross-sectional view of an image forming unit that is usedin the color image forming apparatus according to Embodiment 9 of thepresent invention.

BEST MODE FOR CARRYING OUT THE INVENTION

As described above, the optical scanning device according to the presentinvention includes: a plurality of light sources; a single opticaldeflector that scans light beams emitted respectively from the pluralityof light sources; a first image forming optical system that is disposedbetween the optical deflector and the plurality of light sources andallows linear images of the light beams to be formed on a commondeflecting surface of the optical deflector; and a second image formingoptical system that is disposed between the optical deflector and aplurality of surfaces to be scanned corresponding to the plurality oflight sources and has a plurality of curved surface mirrors that are inone-to-one correspondence with the plurality of surfaces to be scanned.In the optical scanning device, the plurality of light sources, theoptical deflector, and the second image forming optical system aredisposed at different positions in a sub-scanning direction so thatlight beams from the first image forming optical system are incidentrespectively on the deflecting surface obliquely with respect to a planethat includes a normal line at a center of the deflecting surface of theoptical deflector and is parallel to a main scanning direction (mainscanning plane), and so that light beams from the optical deflector areincident respectively on the plurality of curved surface mirrorsobliquely with respect to a plane that includes each of normal lines atvertices of the plurality of curved surface mirrors and is parallel tothe main scanning direction. Further, in the optical scanning device,the plurality of curved surface mirrors are disposed on the same sidewith respect to the main scanning plane. Moreover, in the opticalscanning device, curved surfaces of the plurality of curved surfacemirrors vary in shape.

Herein, “a normal line at a center of the deflecting surface of theoptical deflector” refers to a normal line to a deflecting surface onwhich light beams are incident when the deflecting surface is rotated soas to be in an orientation such that the normal line is included in anXZ plane (a plane including a rotation axis of the optical deflector andthe vertices of the plurality of curved surface mirrors).

According to the above-described optical scanning device of the presentinvention, an optical scanning device can be realized that, while havingoptical paths from light sources to photosensitive members that aredifferent from one another, reduces the number of components, hasexcellent optical performance, and allows a relative difference inperformance between scanning lines to be reduced.

Preferably, in the above-described optical scanning device according tothe present invention, the plurality of curved surface mirrors have awidth in the sub-scanning direction that increases in a direction fromone of the plurality of curved surface mirrors close to the opticaldeflector toward another of the plurality of curved surface mirrors farfrom the optical deflector.

Furthermore, preferably, in the above-described optical scanning deviceaccording to the present invention, in the plane including the rotationaxis of the optical deflector and the vertices of the plurality ofcurved surface mirrors (hereinafter, referred to as the “XZ plane”), notwo from among a plurality of light beams that are incident on theoptical deflector, a plurality of light beams that are reflected off theoptical deflector to be incident on the plurality of curved surfacemirrors, and a plurality of light beams that are reflected off theplurality of curved surface mirrors to be directed toward the pluralityof surfaces to be scanned are parallel to each other.

Furthermore, preferably, in the above-described optical scanning deviceaccording to the present invention, in the plane including the rotationaxis of the optical deflector and the vertices of the plurality ofcurved surface mirrors (XZ plane), a light beam that is incident on thesurface to be scanned farthest from the optical deflector among theplurality of surfaces to be scanned forms an angle of not larger than 20degrees with respect to a light beam that is incident on the surface tobe scanned closest to the optical deflector among the plurality ofsurfaces to be scanned.

Furthermore, preferably, in the above-described optical scanning deviceaccording to the present invention, the plurality of curved surfacemirrors are configured integrally.

Furthermore, preferably, in the above-described optical scanning deviceaccording to the present invention, the plurality of curved surfacemirrors vary in position of the vertices in the sub-scanning direction.

Furthermore, preferably, in the above-described optical scanning deviceaccording to the present invention, in the sub-scanning direction, thevertices of the plurality of curved surface mirrors are at a distancefrom a middle portion in the sub-scanning direction of a correspondingone of the plurality of curved surface mirrors, which increases in adirection from one of the plurality of curved surface mirrors close tothe optical deflector toward another of the plurality of curved surfacemirrors far from the optical deflector.

Furthermore, preferably, in the above-described optical scanning deviceaccording to the present invention, the first image forming opticalsystem includes a single cylindrical lens on which a plurality of thelight beams are incident.

Furthermore, preferably, the above-described optical scanning deviceaccording to the present invention further includes a single aperturethat has a plurality of openings for adjusting shapes of light beamsemitted from the plurality of light sources, and the aperture isdisposed immediately in front of the cylindrical lens.

Furthermore, preferably, in the above-described optical scanning deviceaccording to the present invention, no two from among a plurality oflight beams emitted from the plurality of light sources are parallel toeach other.

Furthermore, preferably, in the above-described optical scanning deviceaccording to the present invention, in the plane including the rotationaxis of the optical deflector and the vertices of the plurality ofcurved surface mirrors (XZ plane), where: among the plurality of curvedsurface mirrors, the curved surface mirror closest to the main scanningplane is a first curved surface mirror, the curved surface mirrorfarthest from the main scanning plane is an N-th (N is an integer notsmaller than 2) curved surface mirror, and the vertex of the firstcurved surface mirror is at a distance Lm from the vertex of the N-thcurved surface mirror; among the plurality of surfaces to be scanned,the surface to be scanned corresponding to the first curved surfacemirror is a first surface to be scanned, the surface to be scannedcorresponding to the N-th curved surface mirror is an N-th surface to bescanned, and an intersection of the first surface to be scanned and anoptical axis of a light beam that is incident on the first surface to bescanned is at a distance Li from an intersection of the N-th surface tobe scanned and an optical axis of a light beam that is incident on theN-th surface to be scanned; the vertex of the N-th curved surface mirroris at a distance D1 from the deflecting surface; and the vertex of theN-th curved surface mirror is at a distance D2 from the intersection ofthe N-th surface to be scanned and the optical axis of the light beamthat is incident on the N-th surface to be scanned,

-   -   a relationship 0.25<(Lm/Li)/(D1/D2)<0.45 is satisfied.

Furthermore, preferably, in the above-described optical scanning deviceaccording to the present invention, in the plane including the rotationaxis of the optical deflector and the vertices of the plurality ofcurved surface mirrors (XZ plane), where: among a plurality of lightbeams that are directed toward the plurality of surfaces to be scanned,the light beam closest to the optical deflector is a first light beam,the light beam farthest from the optical deflector is an N-th (N is aninteger not smaller than 2) light beam, and an optical axis of the firstlight beam forms an angle βr with respect to an optical axis of the N-thlight beam; among the plurality of surfaces to be scanned, the surfaceto be scanned on which the first light beam is incident is a firstsurface to be scanned, the surface to be scanned on which the N-th lightbeam is incident is an N-th surface to be scanned, and an intersectionof the first surface to be scanned and the optical axis of the firstlight beam that is incident on the first surface to be scanned is at adistance Li from an intersection of the N-th surface to be scanned andthe optical axis of the N-th light beam that is incident on the N-thsurface to be scanned; the vertex of an N-th curved surface mirrorcorresponding to the N-th surface to be scanned is at a distance D1 fromthe deflecting surface; and the vertex of the N-th curved surface mirroris at a distance D2 from the intersection of the N-th surface to bescanned and the optical axis of the N-th light beam that is incident onthe N-th surface to be scanned,

-   -   a relationship 1.0<(D1+D2)·tanβr/Li<1.6 is satisfied.

Furthermore, preferably, in the above-described optical scanning deviceaccording to the present invention, in the plane including the rotationaxis of the optical deflector and the vertices of the plurality ofcurved surface mirrors (XZ plane), where: among the plurality of curvedsurface mirrors, the curved surface mirror closest to the main scanningplane is a first curved surface mirror, the curved surface mirrorfarthest from the main scanning plane is an N-th (N is an integer notsmaller than 2) curved surface mirror, among the plurality of surfacesto be scanned, the surface to be scanned corresponding to the firstcurved surface mirror is a first surface to be scanned, the surface tobe scanned corresponding to the N-th curved surface mirror is an N-thsurface to be scanned, and a line linking the vertex of the first curvedsurface mirror with the vertex of the N-th curved surface mirror formsan angle Δβ with respect to a line linking an intersection of the firstsurface to be scanned and an optical axis of a light beam that isincident on the first surface to be scanned with an intersection of theN-th surface to be scanned and an optical axis of a light beam that isincident on the N-th surface to be scanned; the normal line at thevertex of the N-th curved surface mirror forms an angle β2 with respectto an optical axis of an N-th light beam that is incident on the N-thcurved surface mirror from the deflecting surface; the vertex of theN-th curved surface mirror is at a distance D1 from the deflectingsurface; and the vertex of the N-th curved surface mirror is at adistance D2 from the intersection of the N-th surface to be scanned andthe optical axis of the light beam that is incident on the N-th surfaceto be scanned,

-   -   a relationship −1.8<Δβ/β2−0.2 (D1/D2)<0.4 is satisfied.

Furthermore, in the above-described optical scanning device according tothe present invention, in the plane including the rotation axis of theoptical deflector and the vertices of the plurality of curved surfacemirrors (XZ plane), it is assumed that, among a plurality of light beamsthat are directed toward the plurality of surfaces to be scanned, thelight beam closest to the optical deflector is a first light beam, thelight beam farthest from the optical deflector is an N-th (N is aninteger not smaller than 2) light beam, and an optical axis of the firstlight beam forms an angle βr with respect to an optical axis of the N-thlight beam. Further, a plane that is orthogonal to the XZ plane andincludes each of the normal lines at the vertices of the plurality ofcurved surface mirrors is assumed to be a YZ plane in each of theplurality of curved surface mirrors. Further, it is assumed that, amongthe plurality of curved surface mirrors, the curved surface mirrorclosest to the main scanning plane is a first curved surface mirror, andat the vertex of the first curved surface mirror, the first curvedsurface mirror has a radius of curvature RxL in a cross section in theXZ plane and a radius of curvature RyL in a cross section in the YZplane. Further, it is assumed that, among the plurality of curvedsurface mirrors, the curved surface mirror farthest from the mainscanning plane is an N-th curved surface mirror, and at the vertex ofthe N-th curved surface mirror, the N-th curved surface mirror has aradius of curvature RxH in a cross section in the XZ plane and a radiusof curvature RyH in a cross section in the YZ plane. In this case,preferably,

-   -   a relationship 0.001<[1−RyH·RxL/RxH·RyL]/tanβr<0.012 is        satisfied.

Furthermore, preferably, in the above-described optical scanning deviceaccording to the present invention, in the plane including the rotationaxis of the optical deflector and the vertices of the plurality ofcurved surface mirrors (XZ plane), where among the plurality of curvedsurface mirrors, the curved surface mirror closest to the main scanningplane is a first curved surface mirror, the curved surface mirrorfarthest from the main scanning plane is an N-th (N is an integer notsmaller than 2) curved surface mirror, and a line linking anintersection of a first surface to be scanned corresponding to the firstcurved surface mirror and an optical axis of a light beam that isincident on the first surface to be scanned with an intersection of anN-th surface to be scanned corresponding to the N-th curved surfacemirror and an optical axis of a light beam that is incident on the N-thsurface to be scanned forms an angle βid (degree) with respect to anoptical axis of an N-th light beam that is incident on the N-th surfaceto be scanned,

-   -   a relationship 55<βid≦150 is satisfied.

Furthermore, the color image forming apparatus according to the presentinvention includes: the above-described optical scanning deviceaccording to the present invention; a plurality of photosensitivemembers that are disposed respectively on the plurality of surfaces tobe scanned; a plurality of developers that correspond respectively tothe plurality of photosensitive members and develop toner images ofdifferent colors respectively on the plurality of photosensitivemembers; a transferring unit that transfers the toner images on theplurality of photosensitive members onto a transfer material; and afixer that fixes the toner images transferred onto the transfermaterial.

According to the above-described color image forming apparatus of thepresent invention, a small-sized color image forming apparatus can berealized that allows images of excellent quality to be formed at a lowcost.

Hereinafter, the optical scanning device and the color image formingapparatus according to the present invention will be described by way ofspecific embodiments with reference to FIGS. 1 to 14.

Embodiment 1

FIG. 1 is a schematic structural view of an optical unit 40 that is anoptical scanning device in Embodiment 1 of the present invention. In thefollowing description, indices a to d are provided to reference numeralsdenoting elements so as to correspond to four colors (yellow, magenta,cyan, and black) that are used to form color images, while being omittedin the case where there is no need to make a distinction among thecolors.

In FIG. 1, reference characters 42 a to 42 d denote collimating lensesby which light beams emitted from semiconductor lasers 41 a to 41 d thatare a plurality of light sources are converted respectively intoparallel light. Reference characters 43 a to 43 d denote cylindricallenses that have a refractive power only in a direction perpendicular toan optical axis in an XZ plane (sub-scanning direction) and linearlyfocus light beams from the collimating lenses 42 a to 42 d on areflecting surface 46 that is a deflecting surface of a polygon mirror44. Reference character 47 denotes a polygon motor that scans a lightbeam that is incident on the reflecting surface 46 by rotating thepolygon mirror 44 at a predetermined speed. The polygon mirror 44 andthe polygon motor 47 constitute an optical deflector. Further, thecollimating lenses 42 a to 42 d and the cylindrical lenses 43 a to 43 dconstitute a first image forming optical system.

Light beams L1 a to L1 d from the semiconductor lasers 41 a to 41 d areincident respectively on the reflecting surface 46 from an obliquedirection with respect to a plane that includes a normal line to thereflecting surface 46 and is parallel to a main scanning direction (mainscanning plane), and are emitted respectively as light beams L2 a to L2d according to their respective incident angles. The light beams L2 a toL2 d are incident respectively on curved surface mirrors 45 a to 45 dfrom an oblique direction with respect to a plane that includes each ofnormal lines at vertices of reflecting surfaces of the curved surfacemirrors 45 a to 45 d and is parallel to the main scanning direction(this plane is defined with respect to each of the curved surfacemirrors and is referred to as a “YZ plane” of each of the curved surfacemirrors), and are reflected respectively as light beams L3 a to L3 d towhich photosensitive members 4 a to 4 d, which are a plurality ofsurfaces to be scanned, are exposed. All of the curved surface mirrors45 a to 45 d are disposed on the same side (upper side in the figure)with respect to the plane that includes the normal line to thereflecting surface 46 and is parallel to the main scanning direction(main scanning plane). In the XZ plane, no two from among the lightbeams L1 a to L1 d, the light beams L2 a to L2 d, and the light beams L3a to L3 d are parallel to each other. With respect to the shape of eachof the curved surface mirrors 45 a to 45 d, a non-circular arc shape ina cross section in the main scanning direction and a radius of curvaturein the sub-scanning direction corresponding to each image height aredetermined so that curvatures of field in the main and sub-scanningdirections and an f-θ error are compensated, and moreover, an amount ofa skew of each surface of the curved surface mirrors 45 a to 45 d at aposition corresponding to each image height is determined so that acurvature of a scanning line is compensated. As a result, the curvedsurface mirrors 45 a to 45 d vary in shape. These mirrors can be formedof a curved surface mirror such as is described in JP11(1999)-153764 Aor JP2001-100130 A, for example.

In the XZ plane, the light beams L3 a to L3 d have approximately thesame length and are emitted respectively from the curved surface mirrors45 a to 45 d toward the photosensitive members 4 a to 4 d in a fan shapeso as to separate from one another. The distance between optical axes oflight beams toward each pair of adjacent ones of the photosensitivemembers 4 a to 4 d is 25 mm. The light beam L3 a and the light beam L3 dare incident on the photosensitive members 4 a and 4 d so as to beinclined in a vertical direction at an angle of about 8° with respect toa horizontal direction, respectively. That is, in the XZ plane, thelight beam L3 d farthest from the polygon mirror 44 forms an angle of16° with respect to the light beam L3 a closest to the polygon mirror44.

Moreover, the curved surface mirrors 45 a to 45 d are formed integrallyby a method such as resin molding or the like to constitute an integralmirror 51.

FIG. 2 is a front view of the curved surface mirrors 45 a to 45 d.Reference characters 52 a to 52 d denote trajectories of centerpositions of the light beams L2 a to L2 d that are allowed to scan overthe curved surface mirrors 45 a to 45 d, respectively. The light beamsL1 a to L1 d are incident on the reflecting surface 46 from an obliquedirection with respect to the plane that includes the normal line to thereflecting surface 46 and is parallel to the main scanning direction(main scanning plane), so that the trajectories 52 a to 52 d on thecurved surface mirrors 45 a to 45 d form curves as shown in FIG. 2. Thecurves have a curvature that increases with increasing incident angle inthe XZ plane of a corresponding one of the light beams L1 a to L1 d withrespect to the reflecting surface 46, and widths D1 a to D1 d in thesub-scanning direction of the curved surface mirrors 45 a to 45 dcorrespondingly have a relationship D1 a<D1 b<D1 c<D1 d.

The following description is directed to the operation of an opticalscanning device having the above-described configuration with referenceto FIGS. 1 and 2.

Light beams from the semiconductor lasers 41 a to 41 d are turned intoparallel light by the collimating lenses 42 a to 42 d, respectively.Then, the light beams are converged only in the sub-scanning directionby the cylindrical lenses 43 a to 43 d, and are focused as linear imageson the reflecting surface 46 of the polygon mirror 44, respectively. Thepolygon mirror 44 is rotated about a rotational optical axis, so thatthe light beams L1 a to L1 d are allowed to scan to be incident on thecurved surface mirrors 45 a to 45 d as the light beams L2 a to L2 d,respectively. Then, the light beams L2 a to L2 d are reflected by thecurved surface mirrors 45 a to 45 d and form excellent images on thephotosensitive members 4 a to 4 d as the light beams L3 a to L3 d,respectively. With respect to the shape of each of the curved surfacemirrors 45 a to 45 d, a non-circular arc shape in the cross section inthe main scanning direction and a radius of curvature in thesub-scanning direction corresponding to each image height are determinedso that curvatures of field in the main and sub-scanning directions andan f-θ error are compensated. Moreover, an amount of a skew of eachsurface of the curved surface mirrors 45 a to 45 d at a positioncorresponding to each image height is determined so that a curvature ofa scanning line is compensated. Thus, the relative difference inperformance between scanning lines is reduced.

Furthermore, by the curved surface mirrors 45 a to 45 d, light beamsthat are allowed to scan over the photosensitive members 4 a to 4 d arefocused on a photodiode that is disposed at an end portion in a scanningdirection but is not shown. Using detection signals from the photodiodeas synchronizing signals, a controller that is not shown controls thesemiconductor lasers 41 a to 41 d.

The trajectories 52 a to 52 d of the light beams L2 a to L2 d that areallowed to scan over the curved surface mirrors 45 a to 45 d have adegree of curvature that increases with increasing angle in the XZ planeat which a corresponding one of the light beams L2 a to L2 d is emittedfrom the reflecting surface 46. However, since the widths D1 a to Did inthe sub-scanning direction of the curved surface mirrors 45 a to 45 dare in the relationship D1 a<D1 b<D1 c<D1 d according to the respectivedegrees of curvature, an effective reflection region is securedsufficiently in each of the curved surface mirrors, and thus excellentimages are formed on the photosensitive members 4 a to 4 d.

As described above, according to Embodiment 1, curved surfaces of thecurved surface mirrors 45 a to 45 d vary in shape. Thus, even in thecase of an optical scanning device having optical paths from the lightsources 41 a to 41 d to the photosensitive members 4 a to 4 d that aredifferent from one another, an optical scanning device can be realizedthat has excellent optical performance and allows a relative differencein performance between scanning lines to be reduced. Further, it is nolonger necessary to provide a bending mirror between the curved surfacemirrors 45 a to 45 d and the photosensitive members 4 a to 4 d, and thusthe number of components can be reduced.

Furthermore, the curved surface mirrors 45 a to 45 d respectively havethe widths D1 a to D1 d in the sub-scanning direction that are increasedgradually in a direction from the curved surface mirror 45 a close tothe polygon mirror 44 (or the main scanning plane) toward the curvedsurface mirror 45 d far from the polygon mirror 44 (or the main scanningplane). Therefore, an effective reflection region is securedsufficiently in each of the curved surface mirrors, and thus an opticalscanning device can be realized that allows excellent images to beformed on the photosensitive members 4 a to 4 d.

Moreover, in the XZ plane, no two from among the light beams L1 a to L1d that are incident on the reflecting surface 46 of the polygon mirror44, respectively, the light beams L2 a to L2 d that are reflected offthe reflecting surface 46 to be incident on the curved surface mirrors45 a to 45 d, respectively, and the light beams L3 a to L3 d that arereflected off the curved surface mirrors 45 a to 45 d to be directedtoward the photosensitive members 4 a to 4 d, respectively, are parallelto each other. Thus, the optical elements and the photosensitive members4 a to 4 d can be located with an increased degree of freedom, therebyallowing more appropriate characteristics to be obtained.

Moreover, in Embodiment 1, in the XZ plane, the light beam L3 d that isdirected toward the photosensitive member 4 d farthest from the polygonmirror 44 forms an angle (namely, an angle αr that will be describedlater (see FIGS. 7 and 9)) of not larger than 20 degrees with respect tothe light beam L3 a that is directed toward the photosensitive member 4a closest to the polygon mirror 44. In the case of using cylindricalphotosensitive members, each of the photosensitive members has adecentering component and thus is rotated while wobbling around arotation axis. In order to suppress an influence of stray lightoriginating in light reflected off surfaces of the photosensitivemembers, generally, light beams are allowed to be incident on thesurfaces of the photosensitive members at an oblique incident angle withrespect to a normal direction at a position of incidence. Because ofthis, when the photosensitive members are being rotated while wobbling,the positions of incidence of the light beams vary, resulting in theoccurrence of color shifts in a paper-conveying direction. With theabove-described configuration, however, the amount of such color shiftscan be suppressed to a practically permissible level.

In addition, the curved surface mirrors 45 a to 45 d are configured asthe integral mirror 51. Therefore, the number of components can bereduced, and in the case of producing curved surface mirrors by resinmolding or the like, variations in characteristics of the curved surfacemirrors can be suppressed, and thus excellent images free from colorshifts and color unevenness can be obtained.

Embodiment 2

FIG. 3 is a cross-sectional view taken on an XZ plane for showing curvedsurface mirrors 55 a to 55 d in Embodiment 2, and FIG. 4 is a front viewof the curved surface mirrors 55 a to 55 d. The following description isdirected to differences from Embodiment 1. Configurations that are notdescribed specifically are the same as in Embodiment 1.

In FIGS. 3 and 4, the curved surface mirrors 55 a to 55 d are configuredindependently so as to be equal in width in a sub-scanning direction.Reference characters 56 a to 56 d denote trajectories of centerpositions of light beams L2 a to L2 d that are allowed to scan over thecurved surface mirrors 55 a to 55 d, respectively. Light beams Lla toLld are incident on a reflecting surface 46 from an oblique directionwith respect to a plane that includes a normal line to the reflectingsurface 46 and is parallel to a main scanning direction (main scanningplane), so that the trajectories 56 a to 56 d on the curved surfacemirrors 55 a to 55 d form curves as shown in FIG. 4. The curves have acurvature that increases with increasing incident angle in the XZ planeof a corresponding one of the light beams L1 a to L1 d with respect tothe reflecting surface 46. Therefore, the trajectories 56 a to 56 d havea curvature that increases gradually in a direction from the trajectory56 a toward the trajectory 56 d. Reference characters 57 a to 57 ddenote vertices of the curved surface mirrors 55 a to 55 d,respectively. The trajectories 56 a to 56 d are the curves passingthrough the vertices 57 a to 57 d, respectively.

Herein, rectangles 58 a to 58 d that respectively include thetrajectories 56 a to 56 d are defined. That is, the rectangles 58 a to58 d are defined that have one long side whose both ends coincide withboth ends of a corresponding one of the trajectories 56 a to 56 d, andthe other long side whose midpoint coincides with a corresponding one ofmidpoints of the trajectories 56 a to 56 d (namely, the vertices 57 a to57 d). In this embodiment, the rectangles 58 a to 58 d are provided soas to be at substantially a middle in the main scanning direction and inthe sub-scanning direction of a corresponding one of the curved surfacemirrors 55 a to 55 d. That is, the curved surface mirrors 55 a to 55 dvary in position of the vertices 57 a to 57 d in the sub-scanningdirection among the curved surface mirrors 55 a to 55 d, and in thesub-scanning direction, the vertices 57 a to 57 d are at a distance froma middle in the sub-scanning direction of a corresponding one of thecurved surface mirrors 55 a to 55 d, which gradually increases in adirection from the curved surface mirror 55 a close to a polygon mirror44 (or the main scanning plane) toward the curved surface mirror 55 dfar from the polygon mirror 44 (or the main scanning plane).

The following is a description of the operation of an optical scanningdevice having the above-described configuration, in which onlydifferences from Embodiment 1 are described with reference to FIGS. 3and 4.

The trajectories 56 a to 56 d of the light beams L2 a to L2 d that areallowed to scan over the curved surface mirrors 55 a to 55 d have adegree of curvature that increases with increasing angle in the XZ planeat which a corresponding one of the light beams L2 a to L2 d is emittedfrom the reflecting surface 46. However, unlike in Embodiment 1, whenthe rectangles 58 a to 58 d respectively including the trajectories 56 ato 56 d are defined, the rectangles 58 a to 58 d are provided so as tobe at substantially a middle in the main scanning direction and in thesub-scanning direction of a corresponding one of the curved surfacemirrors 55 a to 55 d. That is, in the sub-scanning direction, thevertices 57 a to 57 d of the curved surface mirrors 55 a to 55 d are ata distance from a middle in the sub-scanning direction of acorresponding one of the curved surface mirrors 55 a to 55 d, whichincreases in the direction from the curved surface mirror 55 a close tothe polygon mirror 44 toward the curved surface mirror 55 d far from thepolygon mirror 44. Therefore, even in the configuration in which thecurved surface mirrors 55 a to 55 d are equal in size in the mainscanning direction and in the sub-scanning direction, an effectivereflection region can be secured sufficiently in each of the curvedsurface mirrors, and thus excellent images can be formed onphotosensitive members 4 a to 4 d.

As described above, according to Embodiment 2, the curved surfacemirrors 55 a to 55 d vary in position of the vertices 57 a to 57 d inthe sub-scanning direction. Moreover, in the sub-scanning direction, thevertices are at a distance from a middle in the sub-scanning directionof a corresponding one of the curved surface mirrors 55 a to 55 d, whichincreases in the direction from the curved surface mirror 55 a close tothe polygon mirror 44 (or the main scanning plane) toward the curvedsurface mirror 55 d far from the polygon mirror 44 (or the main scanningplane). Therefore, even in the configuration in which the curved surfacemirrors 55 a to 55 d are equal in size in the main scanning directionand in the sub-scanning direction, an effective reflection region can besecured sufficiently in each of the curved surface mirrors, and thusexcellent images can be formed on the photosensitive members 4 a to 4 d.Thus, in the case of producing the curved surface mirrors 55 a to 55 dby resin molding, molds of the same size can be used, making it easierto adjust molding conditions, thereby allowing variations between thecurved surface mirrors 55 a to 55 d to be reduced.

Embodiment 3

FIG. 5 is a schematic structural view showing a preferred embodiment ofa first image forming optical system that can be used in Embodiment 1 orEmbodiment 2 described above. In FIG. 5, reference characters 62 a to 62d denote collimating lenses by which light beams emitted fromsemiconductor lasers 61 a to 61 d as a plurality of light sources areconverted respectively into parallel light. Reference character 63denotes a single cylindrical lens that has a refractive power only in adirection perpendicular to an optical axis in an XZ plane (sub-scanningdirection) and linearly focuses light beams from the collimating lenses62 a to 62 d on a reflecting surface 66 that is a deflecting surface ofa polygon mirror 64. Light beams emitted from the semiconductor lasers61 a to 61 d are not parallel to one another but have an angle such thatthe light beams are directed inwardly. Reference character 65 denotes anaperture that is formed of one metal plate in which openings 67 a to 67d for adjusting light beams from the collimating lenses 62 a to 62 dinto a predetermined shape are provided by etching, pressing or thelike. The aperture 65 is disposed immediately in front of thecylindrical lens 63.

By the use of the above-described first image forming optical system,while the number of components can be reduced, light beams havinguniform characteristics can be obtained.

Furthermore, the single cylindrical lens 63 is used to form the firstimage forming optical system. This can prevent the occurrence of arelative positional error due to changes over time, and thus stablecharacteristics are obtained.

Moreover, the single aperture 65 in which the openings 67 a to 67 d areformed is disposed immediately in front of the cylindrical lens 63.Thus, compared with the case of using an individual aperture for each ofthe light beams, the number of components can be reduced, and moreover,variations in characteristics due to an error in mounting and aninfluence of changes over time are reduced.

In addition, no two from among light beams from the semiconductor lasers61 a to 61 d are parallel to each other. This can provide an increasedspacing between each pair of adjacent ones of the semiconductor lasers61 a to 61 d, and thus the configuration of a light source block can besimplified.

Embodiment 4

FIG. 6 is a schematic structural view of an optical unit that is anoptical scanning device in Embodiment 4 of the present invention as seenfrom a normal direction to an XZ plane. Like reference charactersindicate like constituent elements with respect to Embodiment 1, forwhich detailed descriptions are omitted.

In this embodiment, as shown in FIG. 6, in a plane including a rotationaxis of a polygon mirror (optical deflector) 44 and vertices of aplurality of curved surface mirrors 45 a to 45 d (XZ plane), where:among the plurality of curved surface mirrors 45 a to 45 d, the curvedsurface mirror closest to a plane that includes a normal line at acenter of a reflecting surface (deflecting surface) 46 and is parallelto a main scanning direction (main scanning plane) is a first curvedsurface mirror 45 a, the curved surface mirror farthest from the mainscanning plane is an N-th (in this embodiment, N=4) curved surfacemirror 45 d, and the vertex of the first curved surface mirror 45 a isat a distance Lm from the vertex of the N-th curved surface mirror 45 d;among a plurality of photosensitive members (surfaces to be scanned) 4 ato 4 d, the photosensitive member corresponding to the first curvedsurface mirror 45 a is a first photosensitive member 4 a, thephotosensitive member corresponding to the N-th curved surface mirror 45d is an N-th photosensitive member 4 d, and an intersection of a surfaceof the first photosensitive member 4 a and an optical axis of a lightbeam L3 a that is incident on the surface of the first photosensitivemember 4 a is at a distance Li from an intersection of a surface of theN-th photosensitive member 4 d and an optical axis of a light beam L3 dthat is incident on the surface of the N-th photosensitive member 4 d;the vertex of the N-th curved surface mirror 45 d is at a distance D1from the reflecting surface (deflecting surface) 46; and the vertex ofthe N-th curved surface mirror 45 d is at a distance D2 from theintersection of the surface of the N-th photosensitive member 4 d andthe optical axis of the light beam L3 d that is incident on the surfaceof the N-th photosensitive member 4 d,

-   -   a relationship 0.25<(Lm/Li)/(D1/D2)<0.45 is satisfied.

When a value of (Lm/Li)/(D1/D2) is lower than a lower limit value of theabove inequality, the spacing between each pair of adjacent ones of theplurality of curved surface mirrors 45 a to 45 d is small, and thuseffective regions off which a plurality of light beams are reflected aresuperimposed on one another, making it difficult to separate the lightbeams. Further, when the value of (Lm/Li)/(D1/D2) is higher than anupper limit value of the above inequality, a curvature of field in themain scanning direction of not less than 2.5 mm is generated. Further, alight beam having an intensity of 1/e² hardly can be reduced in diameterto not more than 80 μm and further to not more than 60 μm, hindering therealization of a resolution of not less than 400 D.P.I.

By referring to FIG. 6, the foregoing description was made using theoptical systems described in Embodiment 1 as an example. However,preferably, the optical systems of Embodiments 2 and 3 also satisfy theabove-mentioned relationship to provide the same effects as describedabove.

Embodiment 5

FIG. 7 is a schematic structural view of an optical unit that is anoptical scanning device in Embodiment 5 of the present invention as seenfrom a normal direction to an XZ plane. Like reference charactersindicate like constituent elements with respect to Embodiment 1, forwhich detailed descriptions are omitted.

In this embodiment, as shown in FIG. 7, in a plane including a rotationaxis of a polygon mirror (optical deflector) 44 and vertices of aplurality of curved surface mirrors 45 a to 45 d (XZ plane), where:among a plurality of light beams L3 a to L3 d that are directed toward aplurality of photosensitive members (surfaces to be scanned) 4 a to 4 d,the light beam closest to the polygon mirror 44 is a first light beam L3a, the light beam farthest from the polygon mirror 44 is an N-th (inthis embodiment, N=4) light beam L3 d, and an optical axis of the firstlight beam L3 a forms an angle βr with respect to an optical axis of theN-th light beam L3 d; among surfaces of the plurality of photosensitivemembers 4 a to 4 d, the surface on which the first light beam L3 a isincident is the surface of a first photosensitive member 4 a, thesurface on which the N-th light beam L3 d is incident is the surface ofan N-th photosensitive member 4 d, and an intersection of the surface ofthe first photosensitive member 4 a and the optical axis of the firstlight beam L3 a that is incident on the surface of the firstphotosensitive member 4 a is at a distance Li from an intersection ofthe surface of the N-th photosensitive member 4 d and the optical axisof the N-th light beam L3 d that is incident on the surface of the N-thphotosensitive member 4 d; the vertex of an N-th curved surface mirror45 d corresponding to the N-th photosensitive member 4 d is at adistance D1 from a reflecting surface (deflecting surface) 46; and thevertex of the N-th curved surface mirror 45 d is at a distance D2 fromthe intersection of the surface of the N-th photosensitive member 4 dand the optical axis of the N-th light beam L3 d that is incident on thesurface of the N-th photosensitive member 4 d,

-   -   a relationship 1.0<(D1+D2)·tanβr/Li<1.6 is satisfied.

When a value of (D1+D2)·tanβr/Li is lower than a lower limit value ofthe above inequality, a curvature of field in a main scanning directionof not less than 2.5 mm is generated. Further, a light beam having anintensity of 1/e² hardly can be reduced in diameter to not more than 80μm and further to not more than 60 μm, hindering the realization of aresolution of not less than 400 D.P.I. Further, when the value of(D1+D2)·tanβr/Li is higher than an upper limit value of the aboveinequality, the spacing between each pair of adjacent ones of theplurality of curved surface mirrors 45 a to 45 d is small, and thuseffective regions off which a plurality of light beams are reflected aresuperimposed on one another, making it difficult to separate the lightbeams.

More preferably,

-   -   1.2<(D1+D2)·tanβr/Li<1.6 is satisfied. When the value of        (D1+D2)·tanβr/Li is lower than a lower limit value of the above        inequality, a curvature of field in the main scanning direction        of not less than 1.0 mm is generated. Further, a light beam        having an intensity of 1/e² hardly can be reduced in diameter to        not more than 60 μm and further to not more than 40 μm,        hindering the realization of a resolution of not less than 600        D.P.I.

By referring to FIG. 7, the foregoing description was made using theoptical systems described in Embodiment 1 as an example. However,preferably, the optical systems of Embodiments 2 and 3 also satisfy theabove-mentioned relationships to provide the same effects as describedabove.

Embodiment 6

FIG. 8 is a schematic structural view of an optical unit that is anoptical scanning device in Embodiment 6 of the present invention as seenfrom a normal direction to an XZ plane. Like reference charactersindicate like constituent elements with respect to Embodiment 1, forwhich detailed descriptions are omitted.

In this embodiment, as shown in FIG. 8, in a plane including a rotationaxis of a polygon mirror (optical deflector) 44 and vertices of aplurality of curved surface mirrors 45 a to 45 d (XZ plane), where:among the plurality of curved surface mirrors 45 a to 45 d, the curvedsurface mirror closest to a plane that includes a normal line at acenter of a reflecting surface (deflecting surface) 46 and is parallelto a main scanning direction (main scanning plane) is a first curvedsurface mirror 45 a, the curved surface mirror farthest from the mainscanning plane is an N-th (in this embodiment, N=4) curved surfacemirror 45 d, among a plurality of photosensitive members (surfaces to bescanned) 4 a to 4 d, the photosensitive member corresponding to thefirst curved surface mirror 45 a is a first photosensitive member 4 a,the photosensitive member corresponding to the N-th curved surfacemirror 45 d is an N-th photosensitive member 4 d, and a line 81 linkingthe vertex of the first curved surface mirror 45 a with the vertex ofthe N-th curved surface mirror 45 d forms an angle AB with respect to aline 82 linking an intersection of a surface of the first photosensitivemember 4 a and an optical axis of a light beam L3 a that is incident onthe surface of the first photosensitive member 4 a with an intersectionof a surface of the N-th photosensitive member 4 d and an optical axisof a light beam L3 d that is incident on the surface of N-thphotosensitive member 4 d; a normal line 49 d at the vertex of the N-thcurved surface mirror 45 d forms an angle β2 with respect to an opticalaxis of an N-th light beam L2 d that is incident on the N-th curvedsurface mirror 45 d from the reflecting surface (deflecting surface) 46;the vertex of the N-th curved surface mirror 45 d is at a distance D1from the reflecting surface (deflecting surface) 46; and the vertex ofthe N-th curved surface mirror 45 d is at a distance D2 from theintersection of the surface of the N-th photosensitive member 4 d andthe optical axis of the light beam L3 d that is incident on the surfaceof the N-th photosensitive member 4 d,

-   -   a relationship −1.8<Δβ/β2−0.2 (D1/D2)<0.4 is satisfied.

When a value of Δβ/β2−0.2 (D1/D2) is lower than a lower limit value ofthe above inequality or higher than an upper limit value of the aboveinequality, a curvature of field in the main scanning direction of notless than 2.5 mm is generated. Further, a light beam having an intensityof 1/e² hardly can be reduced in diameter to not more than 80 μm andfurther to not more than 60 μm, hindering the realization of aresolution of not less than 400 D.P.I.

More preferably,

-   -   a relationship −1.4<Δβ/β2−0.2 (D1/D2)<0 is satisfied. When the        value of Δβ/β2−0.2 (D1/D2) is lower than a lower limit value of        the above inequality or higher than an upper limit value of the        above inequality, a curvature of field in the main scanning        direction of not less than 1.0 mm is generated. Further, a light        beam having an intensity of 1/e² hardly can be reduced in        diameter to not more than 60 μm and further to not more than 40        μm, hindering the realization of a resolution of not less than        600 D.P.I.

Particularly preferably,

-   -   a relationship −0.9<Δβ/β2−0.2 (D1/D2)<−0.5 is satisfied. When        the value of Δβ/β2−0.2 (D1/D2) is lower than a lower limit value        of the above inequality or higher than an upper limit value of        the above inequality, a curvature of field in the main scanning        direction of not less than 0.5 mm is generated. Further, a light        beam having an intensity of 1/e² hardly can be reduced in        diameter to not more than 40 μm and further to not more than 25        μm, hindering the realization of a resolution of not less than        1,200 D.P.I.

By referring to FIG. 8, the foregoing description was made using theoptical systems described in Embodiment 1 as an example. However,preferably, the optical systems of Embodiments 2 and 3 also satisfy theabove-mentioned relationships to provide the same effects as describedabove.

Embodiment 7

FIG. 9 is a schematic structural view of an optical unit that is anoptical scanning device in Embodiment 7 of the present invention as seenfrom a normal direction to an XZ plane. FIG. 10 is a cross-sectionalview taken on the XZ plane for showing curved surface mirrors used inthe optical scanning device according to Embodiment 7. FIG. 11 is across-sectional view taken on a YZ plane (a plane that is orthogonal tothe XZ plane and includes each of normal lines at vertices of the curvedsurface mirrors) for showing the curved surface mirrors. Like referencecharacters indicate like constituent elements with respect toEmbodiments 1 and 2, for which detailed descriptions are omitted.

In this embodiment, as shown in FIG. 9, in a plane including a rotationaxis of a polygon mirror (optical deflector) 44 and vertices of aplurality of curved surface mirrors 55 a to 55 d (XZ plane), it isassumed that, among a plurality of light beams L3 a to L3 d that aredirected toward a plurality of photosensitive members (surfaces to bescanned) 4 a to 4 d, the light beam closest to the polygon mirror 44 isa first light beam L3 a, the light beam farthest from the polygon mirror44 is an N-th (in this embodiment, N=4) light beam L3 d, and an opticalaxis of the first light beam L3 a forms an angle βr with respect to anoptical axis of the N-th light beam L3 d. Further, as shown in FIG. 10,a plane that is orthogonal to the XZ plane and includes each of normallines 59 a to 59 d at vertices 57 a to 57 d of the plurality of curvedsurface mirrors 55 a to 55 d is assumed to be a YZ plane in each of thecurved surface mirrors. Further, it is assumed that, among the pluralityof curved surface mirrors 55 a to 55 d, the curved surface mirrorclosest to a plane that includes a normal line at a center of areflecting surface (deflecting surface) 46 and is parallel to a mainscanning direction (main scanning plane) is a first curved surfacemirror 55 a, and at the vertex 57 a of the first curved surface mirror55 a, the first curved surface mirror 55 a has a radius of curvature RxLin a cross section in the XZ plane (see FIG. 10) and a radius ofcurvature RyL in a cross section in the YZ plane (see FIG. 11). Further,it is assumed that, among the plurality of curved surface mirrors 55 ato 55 d, the curved surface mirror farthest from the main scanning planeis an N-th curved surface mirror 55 d, and at the vertex of the N-thcurved surface mirror 55 d, the N-th curved surface mirror 55 d has aradius of curvature RxH in a cross section in the XZ plane (see FIG. 10)and a radius of curvature RyH in a cross section in the YZ plane (seeFIG. 11). In this case,

-   -   a relationship 0.001<[1−RyH·RxL/RxH·RyL]/tanβr<0.012 is        satisfied.

When a value of [1−RyH·RxL/RxH·RyL]/tanβr is lower than a lower limitvalue of the above inequality or higher than an upper limit value of theabove inequality, a curvature of field in the main scanning direction ofnot less than 2.5 mm is generated. Further, a light beam having anintensity of 1/e² hardly can be reduced in diameter to not more than 80μm and further to not more than 60 μm, hindering the realization of aresolution of not less than 400 D.P.I.

More preferably,

-   -   a relationship 0.003<[1−RyH·RxL/RxH·RyL]/tanβr<0.007 is        satisfied. When the value of [1−RyH·RxL/RxH·RyL]/tanβr is lower        than a lower limit value of the above inequality or higher than        an upper limit value of the above inequality, a curvature of        field in the main scanning direction of not less than 1.0 mm is        generated. Further, a light beam having an intensity of 1/e²        hardly can be reduced in diameter to not more than 60 μm and        further to not more than 40 μm, hindering the realization of a        resolution of not less than 600 D.P.I.

By referring to FIGS. 9 to 11, the foregoing description was made usingthe optical systems described in Embodiment 2 as an example. However,preferably, the optical systems of Embodiments 1 and 3 also satisfy theabove-mentioned relationships to provide the same effects as describedabove.

Embodiment 8

FIG. 12 is a schematic structural view of an optical unit that is anoptical scanning device in Embodiment 8 of the present invention as seenfrom a normal direction to an XZ plane. Like reference charactersindicate like constituent elements with respect to Embodiment 1, forwhich detailed descriptions are omitted.

In this embodiment, as shown in FIG. 12, in a plane including a rotationaxis of a polygon mirror (optical deflector) 44 and vertices of aplurality of curved surface mirrors 45 a to 45 d (XZ plane), where amongthe plurality of curved surface mirrors 45 a to 45 d, the curved surfacemirror closest to a plane that includes a normal line at a center of areflecting surface (deflecting surface) 46 and is parallel to a mainscanning direction (main scanning plane) is a first curved surfacemirror 45 a, the curved surface mirror farthest from the main scanningplane is an N-th (in this embodiment, N=4) curved surface mirror 45 d,and a line 82 linking an intersection of a surface of a firstphotosensitive member (surface to be scanned) 4 a corresponding to thefirst curved surface mirror 45 a and an optical axis of a light beam L3a that is incident on the surface of the first photosensitive member 4 awith an intersection of a surface of an N-th photosensitive member(surface to be scanned) 4 d corresponding to the N-th curved surfacemirror 45 d and an optical axis of a light beam L3 d that is incident onthe surface of the N-th photosensitive member 4 d forms an angle βid(degree) with respect to an optical axis of an N-th light beam L3 d thatis incident on the N-th photosensitive member 4 d,

-   -   a relationship 55<βid≦150 is satisfied.

This configuration can prevent a phenomenon in which a reflection regionof the N-th curved surface mirror 45 d farthest from the main scanningplane blocks a light beam L3 c that is directed from the curved surfacemirror 45 c adjacent to the N-th curved surface mirror 45 d toward aphotosensitive member 4 c and a phenomenon in which a reflection regionof the first curved surface mirror 45 a closest to the main scanningplane blocks a light beam L2 b that is directed toward the curvedsurface mirror 45 b adjacent to the first curved surface mirror 45 a.Thus, excellent optical performance can be secured, and a relativeperformance error in scanning lines can be reduced, thereby allowinghigh resolution to be achieved.

By referring to FIG. 12, the foregoing description was made using theoptical systems described in Embodiment 1 as an example. However,preferably, the optical systems of Embodiments 2 and 3 also satisfy theabove-mentioned relationship to provide the same effects as describedabove.

Embodiment 9

FIG. 13 is a schematic cross-sectional view showing a color imageforming apparatus to which any one of the optical scanning devicesaccording to Embodiments 1 to 8 is applied. In FIG. 13, referencecharacters 2 a to 2 d denote image forming units corresponding to fourcolors (yellow, magenta, cyan, and black), respectively.

FIG. 14 is a cross-sectional view of the image forming units 2 a to 2 d.In FIG. 14, since the image forming units have the same configuration,only one of the image forming units is shown with indices omitted.Reference character 9 denotes a photosensitive drum as a surface to bescanned that has a surface covered with a photosensitive member whosecharges are changed by irradiation with light, and reference character10 denotes a charging roll that attaches electrostatic ions to a surfaceof the photosensitive member so that the photosensitive member ischarged. Reference character 11 denotes a developing unit that attachescharged toner to an electrostatic latent image portion formed on thephotosensitive drum 9, and reference character 12 denotes a transferroll that transfers a toner image formed on the photosensitive drum 9onto a transfer material (paper sheet) 30. The image forming unit 2 iscomposed of the photosensitive drum 9, the charging roll 10, thedeveloping unit 11, and the transfer roll 12.

In FIG. 13, reference characters 14 and 15 denote a fixer that fixestransferred toner on a paper sheet and a paper-feeding cassette,respectively. Further, reference characters 16, 17 and 18 denote any oneof the optical scanning devices described in Embodiments 1 to 8, a lightsource block that is composed of a semiconductor laser, an axialsymmetric lens and a cylindrical lens, and a polygon mirror,respectively, and reference characters 20 a to 20 d denote curvedsurface mirrors. FIG. 13 shows an example in which the curved surfacemirrors 20 a to 20 d are configured integrally as in Embodiment 1.However, a configuration also can be employed in which the curvedsurface mirrors 20 a to 20 d are separated as in Embodiment 2.

The image forming units 2 a to 2 d that correspond respectively to thefour colors (yellow, magenta, cyan, and black) are disposed in alongitudinal direction. Electrostatic latent images corresponding to therespective colors are formed on the photosensitive drums 9 a to 9 d anddeveloped by the developing units 11 a to 11 d, respectively. Thedeveloped toner images are transferred sequentially for each color ontoa paper sheet that has been conveyed from the paper-feeding cassette 15by means of the transfer rolls 12 a to 12 d and fixed by the fixer 14.

According to this configuration, a small-sized color image formingapparatus can be realized that achieves low cost, high-speed operationand high resolution.

As described above, according to this embodiment, a paper-conveying pathis provided in a perpendicular direction, and the image forming units 2a to 2 d are arranged so as to be stacked in the longitudinal direction.Therefore, by setting the size of a housing to be reduced in a verticaldirection, and further by placing the paper-feeding cassette 15 on alower side of the image forming units 2 a to 2 d so as to eliminate thedisadvantage that setting space increases due to the paper-feedingcassette 15 projecting in a lateral direction, a compact apparatus canbe realized easily. That is, with respect to a conventionalconfiguration in which four single-color optical units are disposed soas to be stacked in a longitudinal direction, according to thisembodiment, a single optical unit is used, and a position at which eachof laser beams for the respective colors forms an image can be adjustedfreely. Thus, even in the configuration in which the image forming units2 a to 2 d are arranged in four layers in the longitudinal direction, anincrease in size in the vertical direction is avoided.

Furthermore, light beams L3 a to L3 d that are directed from the curvedsurface mirrors 20 a to 20 d toward the photosensitive drums 9 a to 9 ddiffuse in substantially a fan shape in an XZ plane, and thus thespacing between each pair of adjacent ones of the curved surface mirrors20 a to 20 d can be smaller than the spacing between each pair ofadjacent ones of the photosensitive drums 9 a to 9 d, thereby allowingthe accuracy of components to be secured. The respective angles of lightbeams L1 a to L1 d, light beams L2 a to L2 d, and the light beams L3 ato L3 d can be set freely, and thus any arrangement can be selected soas to be suited for each apparatus. However, preferably, as in thisembodiment, the spacing between each pair of adjacent ones of the curvedsurface mirrors 20 a to 20 d is made small, and the curved surfacemirrors 20 a to 20 d are configured integrally by resin molding or thelike.

Furthermore, in the XZ plane, the light beam L3 a and the light beam L3d are set to form an angle of 16°. In this case, even if thephotosensitive drums 9 a to 9 d have a decentering component of 100 μm,color shifts due to the decentering component can be suppressed to anamount of not more than 30 μm.

The smaller the angle the light beam L3 a and the light beam L3 d form,the smaller the amount of color shifts. However, the closer the lightbeam L3 a and the light beam L3 d come to a parallel state, the largerthe required spacing between each pair of adjacent ones of the curvedsurface mirrors 20 a to 20 d becomes. Because of this, in some cases,the curved surface mirrors 20 a to 20 d hardly can be configuredintegrally, and the optical scanning device 16 is increased in size. Inother cases, the spacing between each pair of adjacent ones of thephotosensitive drums 9 a to 9 d becomes too small, making it difficultto arrange the developing units 11 a to 11 d, the charging rolls 10 a to10 d and the like. Therefore, preferably, the light beams L3 have alength 10 or less times greater than the spacing between each pair ofadjacent ones of the light beams L3 at the respective positions at whichthe light beams L3 are incident on the photosensitive drums 9 a to 9 d.

Furthermore, the color image forming apparatus according to thisembodiment was operated continuously for a long time to find thatexcellent images were obtained without particularly presenting a problemof, for example, the deterioration in image quality. This was based onthe following configurations. That is, a second image forming opticalsystem is composed only of the curved surface mirrors 20 a to 20 d, andthus unlike the case of an optical system using a lens, there are noissues of a refractive index changing due to a temperature change.Further, with respect to the fixer 14, the curved surface mirrors 20 ato 20 d are disposed so as to be farther than the photosensitive drums 9a to 9 d and the polygon mirror 18, and thus are far from the fixer 14that is a heat source, thereby reducing the degree of deformation of thecurved surface mirrors 20 a to 20 d.

Furthermore, according to this configuration, by changing theconfiguration of an inner portion of the optical scanning device 16 (forexample, by changing normal directions of the curved surface mirrors),the spacing between each pair of adjacent ones of laser beams of therespective colors can be adjusted freely, and thus the spacing betweeneach pair of adjacent ones of the image forming units 2 a to 2 d can bereduced. Further, the spacing between each pair of adjacent ones ofcurved surface mirrors 20 a to 20 d can be made smaller than the spacingbetween each pair of adjacent ones of the photosensitive drums 9 a to 9d, and thus high mounting accuracy can be maintained. In theabove-described technical configuration, preferably, in view of mountingworkability and the like, the image forming units 2 a to 2 d are formedinto cartridges, each containing a corresponding one of thephotosensitive drums 9 a to 9 d and as many peripheral components aspossible.

The embodiments disclosed in this application are intended to illustratethe technical aspects of the invention and not to limit the inventionthereto. The invention may be embodied in other forms without departingfrom the spirit and the scope of the invention as indicated by theappended claims and is to be broadly construed.

1. An optical scanning device, comprising: a plurality of light sources;a single optical deflector that scans light beams emitted respectivelyfrom the plurality of light sources; a first image forming opticalsystem that is disposed between the optical deflector and the pluralityof light sources and allows linear images of the light beams to beformed on a common deflecting surface of the optical deflector; and asecond image forming optical system that is disposed between the opticaldeflector and a plurality of surfaces to be scanned corresponding to theplurality of light sources and has a plurality of curved surface mirrorsthat are in one-to-one correspondence with the plurality of surfaces tobe scanned, wherein the plurality of light sources, the opticaldeflector, and the second image forming optical system are disposed atdifferent positions in a sub-scanning direction so that light beams fromthe first image forming optical system are incident respectively on thedeflecting surface obliquely with respect to a plane that includes anormal line at a center of the deflecting surface of the opticaldeflector and is parallel to a main scanning direction (hereinafter,referred to as a “main scanning plane”), and so that light beams fromthe optical deflector are incident respectively on the plurality ofcurved surface mirrors obliquely with respect to a plane that includeseach of normal lines at vertices of the plurality of curved surfacemirrors and is parallel to the main scanning direction, the plurality ofcurved surface mirrors are disposed on the same side with respect to themain scanning plane, and curved surfaces of the plurality of curvedsurface mirrors vary in shape.
 2. The optical scanning device accordingto claim 1, wherein the plurality of curved surface mirrors have a widthin the sub-scanning direction that increases in a direction from one ofthe plurality of curved surface mirrors close to the optical deflectortoward another of the plurality of curved surface mirrors far from theoptical deflector.
 3. The optical scanning device according to claim 1,wherein in a plane including a rotation axis of the optical deflectorand the vertices of the plurality of curved surface mirrors, no two fromamong a plurality of light beams that are incident on the opticaldeflector, a plurality of light beams that are reflected off the opticaldeflector to be incident on the plurality of curved surface mirrors, anda plurality of light beams that are reflected off the plurality ofcurved surface mirrors to be directed toward the plurality of surfacesto be scanned are parallel to each other.
 4. The optical scanning deviceaccording to claim 1, wherein in a plane including a rotation axis ofthe optical deflector and the vertices of the plurality of curvedsurface mirrors, a light beam that is incident on the surface to bescanned farthest from the optical deflector among the plurality ofsurfaces to be scanned forms an angle of not larger than 20 degrees withrespect to a light beam that is incident on the surface to be scannedclosest to the optical deflector among the plurality of surfaces to bescanned.
 5. The optical scanning device according to claim 1, whereinthe plurality of curved surface mirrors are configured integrally. 6.The optical scanning device according to claim 1, wherein the pluralityof curved surface mirrors vary in position of the vertices in thesub-scanning direction.
 7. The optical scanning device according toclaim 1, wherein in the sub-scanning direction, the vertices of theplurality of curved surface mirrors are at a distance from a middleportion in the sub-scanning direction of a corresponding one of theplurality of curved surface mirrors, which increases in a direction fromone of the plurality of curved surface mirrors close to the opticaldeflector toward another of the plurality of curved surface mirrors farfrom the optical deflector.
 8. The optical scanning device according toclaim 1, wherein the first image forming optical system comprises asingle cylindrical lens on which a plurality of the light beams areincident.
 9. The optical scanning device according to claim 8, wherein asingle aperture further is provided that has a plurality of openings foradjusting shapes of light beams emitted from the plurality of lightsources, and the aperture is disposed immediately in front of thecylindrical lens.
 10. The optical scanning device according to claim 1,wherein no two from among a plurality of light beams emitted from theplurality of light sources are parallel to each other.
 11. The opticalscanning device according to claim 1, wherein in a plane including arotation axis of the optical deflector and the vertices of the pluralityof curved surface mirrors, where: among the plurality of curved surfacemirrors, the curved surface mirror closest to the main scanning plane isa first curved surface mirror, the curved surface mirror farthest fromthe main scanning plane is an N-th (N is an integer not smaller than 2)curved surface mirror, and the vertex of the first curved surface mirroris at a distance Lm from the vertex of the N-th curved surface mirror;among the plurality of surfaces to be scanned, the surface to be scannedcorresponding to the first curved surface mirror is a first surface tobe scanned, the surface to be scanned corresponding to the N-th curvedsurface mirror is an N-th surface to be scanned, and an intersection ofthe first surface to be scanned and an optical axis of a light beam thatis incident on the first surface to be scanned is at a distance Li froman intersection of the N-th surface to be scanned and an optical axis ofa light beam that is incident on the N-th surface to be scanned; thevertex of the N-th curved surface mirror is at a distance D1 from thedeflecting surface; and the vertex of the N-th curved surface mirror isat a distance D2 from the intersection of the N-th surface to be scannedand the optical axis of the light beam that is incident on the N-thsurface to be scanned, a relationship 0.25<(Lm/Li)/(D1/D2)<0.45 issatisfied.
 12. The optical scanning device according to claim 1, whereinin a plane including a rotation axis of the optical deflector and thevertices of the plurality of curved surface mirrors, where: among aplurality of light beams that are directed toward the plurality ofsurfaces to be scanned, the light beam closest to the optical deflectoris a first light beam, the light beam farthest from the opticaldeflector is an N-th (N is an integer not smaller than 2) light beam,and an optical axis of the first light beam forms an angle βr withrespect to an optical axis of the N-th light beam; among the pluralityof surfaces to be scanned, the surface to be scanned on which the firstlight beam is incident is a first surface to be scanned, the surface tobe scanned on which the N-th light beam is incident is an N-th surfaceto be scanned, and an intersection of the first surface to be scannedand the optical axis of the first light beam that is incident on thefirst surface to be scanned is at a distance Li from an intersection ofthe N-th surface to be scanned and the optical axis of the N-th lightbeam that is incident on the N-th surface to be scanned; the vertex ofan N-th curved surface mirror corresponding to the N-th surface to bescanned is at a distance D1 from the deflecting surface; and the vertexof the N-th curved surface mirror is at a distance D2 from theintersection of the N-th surface to be scanned and the optical axis ofthe N-th light beam that is incident on the N-th surface to be scanned,a relationship 1.0<(D1+D2)·tanβr/Li<1.6 is satisfied.
 13. The opticalscanning device according to claim 1, wherein in a plane including arotation axis of the optical deflector and the vertices of the pluralityof curved surface mirrors, where: among the plurality of curved surfacemirrors, the curved surface mirror closest to the main scanning plane isa first curved surface mirror, the curved surface mirror farthest fromthe main scanning plane is an N-th (N is an integer not smaller than 2)curved surface mirror, among the plurality of surfaces to be scanned,the surface to be scanned corresponding to the first curved surfacemirror is a first surface to be scanned, the surface to be scannedcorresponding to the N-th curved surface mirror is an N-th surface to bescanned, and a line linking the vertex of the first curved surfacemirror with the vertex of the N-th curved surface mirror forms an angleAB with respect to a line linking an intersection of the first surfaceto be scanned and an optical axis of a light beam that is incident onthe first surface to be scanned with an intersection of the N-th surfaceto be scanned and an optical axis of a light beam that is incident onthe N-th surface to be scanned; the normal line at the vertex of theN-th curved surface mirror forms an angle β2 with respect to an opticalaxis of an N-th light beam that is incident on the N-th curved surfacemirror from the deflecting surface; the vertex of the N-th curvedsurface mirror is at a distance D1 from the deflecting surface; and thevertex of the N-th curved surface mirror is at a distance D2 from theintersection of the N-th surface to be scanned and the optical axis ofthe light beam that is incident on the N-th surface to be scanned, arelationship −1.8<Δβ/β2–0.2 (D1/D2)<0.4 is satisfied.
 14. The opticalscanning device according to claim 1, wherein in a plane including arotation axis of the optical deflector and the vertices of the pluralityof curved surface mirrors (hereinafter, referred to as an “XZ plane”),where: among a plurality of light beams that are directed toward theplurality of surfaces to be scanned, the light beam closest to theoptical deflector is a first light beam, the light beam farthest fromthe optical deflector is an N-th (N is an integer not smaller than 2)light beam, and an optical axis of the first light beam forms an angleαr with respect to an optical axis of the N-th light beam; a plane thatis orthogonal to the XZ plane and includes each of the normal lines atthe vertices of the plurality of curved surface mirrors is a YZ plane ineach of the plurality of curved surface mirrors; among the plurality ofcurved surface mirrors, the curved surface mirror closest to the mainscanning plane is a first curved surface mirror, and at the vertex ofthe first curved surface mirror, the first curved surface mirror has aradius of curvature RxL in a cross section in the XZ plane and a radiusof curvature RyL in a cross section in the YZ plane; and among theplurality of curved surface mirrors, the curved surface mirror farthestfrom the main scanning plane is an N-th curved surface mirror, and atthe vertex of the N-th curved surface mirror, the N-th curved surfacemirror has a radius of curvature RxH in a cross section in the XZ planeand a radius of curvature RyH in a cross section in the YZ plane, arelationship 0.001<[1−RyH·RxL/RxH·RyL]/tanβr<0.012 is satisfied.
 15. Theoptical scanning device according to claim 1, wherein in a planeincluding a rotation axis of the optical deflector and the vertices ofthe plurality of curved surface mirrors, where among the plurality ofcurved surface mirrors, the curved surface mirror closest to the mainscanning plane is a first curved surface mirror, the curved surfacemirror farthest from the main scanning plane is an N-th (N is an integernot smaller than 2) curved surface mirror, and a line linking anintersection of a first surface to be scanned corresponding to the firstcurved surface mirror and an optical axis of a light beam that isincident on the first surface to be scanned with an intersection of anN-th surface to be scanned corresponding to the N-th curved surfacemirror and an optical axis of a light beam that is incident on the N-thsurface to be scanned forms an angle βid (degree) with respect to anoptical axis of an N-th light beam that is incident on the N-th surfaceto be scanned, a relationship 55<βid≦150 is satisfied.
 16. A color imageforming apparatus, comprising: an optical scanning device as claimed inclaim 1; a plurality of photosensitive members that are disposedrespectively on the plurality of surfaces to be scanned; a plurality ofdevelopers that correspond respectively to the plurality ofphotosensitive members and develop toner images of different colorsrespectively on the plurality of photosensitive members; a transferringunit that transfers the toner images on the plurality of photosensitivemembers onto a transfer material; and a fixer that fixes the tonerimages transferred onto the transfer material.